Seismometer



Jan. 12, 11943. H. HOOVER, JR 2,307,792

SEISMOMETER Filed July 28, 1939 3 Sheets-Sheet.l

Escapes/2 A MPL IF! ER & a Peta/e052 INVENTOR, Haves ER T Hoot/2 Jan m,1943. H. HOOVER. JR

SEISMOMETER Filed July 28, 1939 3 Sheets-Sheet 2 IN! 'ENTOR. H5255? 7'H00 v52 J2.

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Jan. LAY 1943. HQOVER JR $97,792

SEISMOMETER Filed July 28, 1959 3 Sheets-Sheet 3 IN VEN TOR.

HERBERT H00 l/EEJQ.

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ulenled darn it 143 SEIISIWWTE Herbert Hoover, in, Sierra e, lcalilt,assignor to llolidated Engineering (Corporation, Pasadena, l3., acorporation oi California Application Jl'y 2d, 1939, aerial No. 287,0'73

4i lms. (Cl. 177-352) invention relates to seismic exploration.

In exploring the earth by the seismic method, seismic waves are usuallyproduced by detonating an explosive charge at or near the surface of theearth. Some of the waves so generated travel 5 downward in the earth andare reflected upward by layers at various depths. The reflected wavesreturning to the surface of the earth reach seismometers which arepositioned at various spaced points from the generating point, and areconverted by the seisrnometers into electrical voltage waves ofcorresponding amplitude and frequency. The electrical waves received ateach seismometer are then amplified and. recorded. The records producedby this method usually contain a plurality of oscillograph tracesgenerally extending in a direction corresponding to the elapse of timeand have displacements in a direction perpendicular to the time axis.Such displaceme'n represents the instantaneous motion of the ear j O atthe point where the corresponding scismometer is located during thereception ofthe seismic wave.

en a wave reaches a seismometer sensitive to movements along an axissuch as the vertical, 2 earth movements in one direction, say upward,produce voltages of one sign while earth movements in the oppositedirection produce voltages oi the opposite sign. These are recorded asoscillograph trace displacements in opposite directions.

Sometimes a seisrnometer is placed at the surlace oi the earth with itsaxis pointing upward, and sometimes with its axis pointing downward.Under these different circumstances, trace displacements a givendirection on the record sometimes represent upward movement, of theearth, and at other times may represent downward movement of the earth.Such variations in the sense relationship between the earth movement andthe trace displacement are quite often dilhcult to detect, and mayresult in errors in interpretation. It is frequently also important tolrnow the direction of earth movement.

l-hccordingly, the object of my invention is to provide for recording aseries at seismic waves received at difierent points or at diderenttimes at same point, in which the direction of trace displacement on therecord always bears the same sense relationship to ground movement, re-50 gardless of which oi two seismometer orientations may he in use atthe time the respective ground movements are being detected.

Other objects of my invention will be apparent or will he specificallypointed out in the description forming a part of this specification, butI do not limit myself to the embodiment of the invention hereindescribed, as various forms may be adopted within the scope of theclaims.

In order to accomplish the object of my invention, I utilize some formof adjustable signal inversion means such as a reversing switchconnected in the electrical circuit between the seismometer and theamplifier, recorder or other wave reproducer. To produce records inwhich the trace displacement always bears the same sense relationship toground movement regardless oi' which of two opposite plantingorientations may be assumed for the seismometer, I record waves withoutinversion for one orientation, and record the waves with inversion whenthe ',/other seismometer orientation is in use.

Referring to the drawings:

Fig. 1 is a schematic view of my seismometer planted on the surface oftheearth, together with an associated seismic wave reproducing channel.

Fig. 2 is a schematicyiew of my seismometer planted beneath the suriaceof the earth together with the associated seismic wave reproducingchannel.

Fig. 3 is a central vertical section of the interior of one embodimentof my invention.-

Fig. 4 is a plan view partly in section taken along the line lll of Fig.3'.

Fig. 5 is a bottom view or the base plate of my seismometer as viewedalong line 5-5 of Fig. 3.

Fig. 6 is a detail bottom view of my seismometer with base plate andcover plate removed and showing one method of mounting a moving coil inmy seismometer.

As illustrated in Figs. 1, 2 and 3 the general external features of myseismometer l are a flat surface on one end i, a dome shape on the otherend ll, a carrying rope l, and a cable 5. Said carrying rope and cableare attached to the seismometer adjacent its flat end so as'to 1 ermltsolid contact oithe fiat end 2 when planted on the surface t of theearth l and to ofi'er a minimum ount of wind resistance when thusplanted, as shown in Fig. 1. When planted in this manner my seismometeris subject to a minimum amount of wind disturbance partly because therope it and cable a lie in direct contact with the earth except for avery short portion next to the seismometer case and partly because windcurrents readily flow over the smooth dome l.

When I desire to plant my seismometer beneath the earths surface I dig ahole which has a dome shaped or round bottom, place the dome end of myseismometer in contact with said bottom so as to form a firm contactwith the soil and cover the seismometer with soil thus froming a plantas shown in Fig. 2. It is clear that when planted beneath the surface inthis manner my seismometer provides firm contact with the ground at allpoints and disturbances due to any vibration of loose soil are avoided.

As is well known in the art, a seismometer converts earth motion intocorresponding electromative forces which are amplified and recorded byapparatus symbolized by ill in Figs. 1 and 2. The records produced areoscillograph traces which accordingly represent ground motion. In orderto provide a one to one correlation between the direction of groundmovement and the direction of displacement of such an oscillograph traceI utilize a reversing switch H to reverse the connections of cable 5 andthe amplifier I when I change from a surface plant of Fig. 1 to asubsurface plant of Fig. 2.

Referring to Fig. 3, the dome shaped cover 3. forms an inset machine fit20 with the main base plate 2|; and 22 is a cover plate. Said dome 3,base plate 2|, and cover plate 22 are made of non-magnetic material.Within the casing formed by the dome 3 and plate 2| is mounted anannular permanent magnet 23 having at one end an annular soft iron endplate 22 to which an annular soft iron pillar 25 is secured by a pressfit, and at the other end an annular soft iron ring 21. secured to saidannular magnet 23 by screws 28. An air gap is formed between the ring 21and pillar 25 as shown in such a manner that magnetic fiux flows throughthe gap radially and is substantially uniform throughout the width ofthe gap. An annular moving coil 29 comprising a copper winding in abrass cylindrical spool is flexibly suspended in said air gap. Thecopper winding itself is preferably longer than the vertical length ofthe air gap for reasons which will be pointed out below. As shown inFigs. 3 and 6 said coil 29 is mounted in spool 30-30 and the wholesuspended from the end ring 2'? by means of two spider type suspensionseach comprising a set of springs 3 l, 32, and 33.

Referring to Fig. 4 the dome 3 is secured to plate 2i by screws 32.Leads Al and 42 connected respectively to the winding of coil 29 atsprin mounts BI and 22 are connected to damping resistor 113.

As illustrated in Fig. 5 said leads ll and 22 pass through holes it-44in base plate 2! and conjoint 20 and by filling holes 48, M, and M withahigh melting point asphalt or wax. Cover plate 22 serves to holdcarrying rope t to the sides of the case and also protects the portionof cable 5 and leads lll and 42 which lie in V-shaped slot 63. Saidcover plate 22 and base plate 2| make possible easy replacement of saidcarrying rope 2 without breaking the seal of said seismometer.

Said ring 21 and said end plate 24 are In order to obtain substantiallyuniform sensitivity in either seismometer plant shown in Figs. 1 or 2, Iuse a dynamic type instrument as illustrated in which the mass of themoving coil is small and the natural period of said coil falls in theregion of the lowest frequency seismic waves I expect to record. Such asuitable natural frequency lies at about 15 or 20 cycles. With a coilhaving such characteristics, the winding will cut almost all the fluxpassing through the air gap inboth positions shown in Fig. 2 and alsowhen the seismometer lies on its side or any other position.

I claim:

1. In apparatus for seismic surveying a seismometer having oppositeground planting orientations making ground contact with seismometersurfaces of different contour, said seismometer having substantiallyequal sensitivity to vibratory earth motion in either of the twoopposite planting orientations, said seismometer being adapted togenerate electrical vibratory signals corresponding to the vibratoryearth motion in either orientation. the electrical signals generated inthe two orientations being of opposite polarity, a vibration recorderelectrically connected to the seismometer, and a reversing switchconnected intermediate the seismometer and the recorder adapted tocontrol the sense of the recorded vibration to facilitate recording theelectrical waves with the same sense relationship to the received earthmotion irrespective of the planting orientation of said seismometer.

2. In making a series of seismic vibration records in the course ofseismic prospecting involving the planting of a seismometer in the earthin any one of a number of predetermined orientations, receiving seismicwaves at the seismometer, converting the seismic waves intocorresponding electrical waves in which the sense relationship of theelectrical waves to the received seismic waves depends on theorientation in which the seismometer is planted, and recording theelectrical waves, the improvement which comprises converting at leastone received seismic Wave into an electrical wave bearing one senserelationship to the received seismic wave, converting each of theremaining received seismic waves into an electrical wave bearing theopposite sense relationship to these remaining received waves, directlyrecording electrical waves bearing one sense relationship to thereceived seismic waves, and reversing the polarity of the otherelectrical waves prior to recording so as to produce a series of recordsin which the recorded waves always bears a fixed sense relationship tothe received seismic waves.

3. Apparatus for seismic surveying comprising a seismometer havingground contact surfaces of differing. contour and having substantiallyequal sensitivity to ground motion in either of two opposite plantingorientations in each of which one of said surfaces is beneath the otherand in contact with the ground. a seismic wave amplifier connected tothe seismometer, a reproducer connected to the output of the amplifier,and signal inversion means connected in the circuit interconnecting theseismometer and the reproducer to provide the same sense relationship ofthe reproduced wave to the receive earth motion irrespective of which ofthe two planting orientations is utilized during the detection of theearth motion.

4. Apparatus for seimic surveying comprising a seismometer havingopposite ground contact surfaces of differing contour and havingsubstantially equal sensitivity in either of two opposite plantingorientations assumed by said seismometer when one of said surfaces is incontact with the ground and beneath the other, a seismic wave amplifierconnected to said seismometer, a i'eproducer connected to the output ofsaid amplifier, and means for inverting the sense of signals passingthrough said connections in accordance with seismometer orientation toprovide the same sense relationship of the repro duced wave to threceived earth motion irre- 5 spective of the seismometer plantingorientation in use at the time of the wave reception.

HERBERT HOOVER, JR.

